Fix settings by making key filterable in meilisearc

lib/meilisearch.dart

@@ -142,6 +142,11 @@ Future<void> init() async {
       body: jsonEncode({'uid': settingsIndex, 'primaryKey': 'key'}),
     );
   }
+  await http.put(
+    Uri.parse('$endpoint/indexes/$settingsIndex/settings/filterable-attributes'),
+    headers: headers,
+    body: jsonEncode(['key', 'value']),
+  );
 }
 
 Future<bool> indexExists(String index) async {